Microlens array architectures for enhanced light outcoupling from an OLED array

ABSTRACT

Novel microlens array architectures for enhanced light outcoupling from light emission are provided. Organic light emitting devices (OLEDs) that include an outcoupling layer including these novel microlens array architectures and method for fabricating such OLEDs are provided. These devices may be used to provide OLEDs with optimized light extraction.

PRIORITY

This application claims priority to U.S. Provisional Application No.61/831,359, filed Jun. 5, 2013, the disclosure of which is incorporatedby reference in its entirety.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Regents of the University of Michigan,Princeton University, The University of Southern California, Kent StateUniversity, and the Universal Display Corporation. The agreement was ineffect on and before the date the claimed invention was made, and theclaimed invention was made as a result of activities undertaken withinthe scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to devices involving light emission andlight absorption such as light-emitting diodes (LEDs), organiclight-emitting devices (OLEDs), and photovoltaic devices (PVs). Morespecifically, it relates to microlens array architectures for enhancedlight outcoupling from light emission.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond fromnitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed thatthe ligand directly contributes to the photoactive properties of anemissive material. A ligand may be referred to as “ancillary” when it isbelieved that the ligand does not contribute to the photoactiveproperties of an emissive material, although an ancillary ligand mayalter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled inthe art, a first work function is “greater than” or “higher than” asecond work function if the first work function has a higher absolutevalue. Because work functions are generally measured as negative numbersrelative to vacuum level, this means that a “higher” work function ismore negative. On a conventional energy level diagram, with the vacuumlevel at the top, a “higher” work function is illustrated as furtheraway from the vacuum level in the downward direction. Thus, thedefinitions of HOMO and LUMO energy levels follow a different conventionthan work functions.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

Extraction of light from the substrate of a light emitting device is amajor challenge in solid state lighting. When light crosses from amaterial with index of refraction no to a material with index n₁, suchthat n₀>n₁, a portion of the incident light rays remain in the higherindex material due to total internal reflection. Glass and plastic havesignificantly higher refractive indices than air and light emittingstructures have indices that are higher still. The potential for lightloss therefore exists at two interfaces in an OLED structure. Ingeneral, only about 25% of light generated in the emissive layer of anOLED is outcoupled to the surrounding environment. Typically, light islost to total internal reflection at both the interface between thedevice and its substrate and the substrate to air interface.

Microlens arrays can be applied to the light emitting side of OLEDsubstrates to reduce the light lost due to total internal reflection atthe substrate to air interface. (Moller et al., Improved LightOut-coupling in Organic Light Emitting Diodes Employing OrderedMicrolens Arrays, J. Appl. Phys. 91, 3324 (2002)). Microlens arrays arewell known outcoupling aids for OLED lighting. Existing examples ofmicrolens arrays feature arrays of microlenses of uniform size with aspherical thickness profile. Microlens arrays are currently massproduced as outcoupling aids for devices such as LCD displays and aregenerally fabricated from plastic with a relatively low index ofrefraction, such as n˜1.5. Most lenses have faces with constantcurvature for ease of manufacturing although aspheric lenses are used inhigh end imaging optics to reduce spherical aberration. Microlenses aregenerally spherical and the shape of molded or embossed microlenses isdetermined by the shape of the mold used to form them. Asphericalmicrolenses can be readily formed from molds with aspherical features,however these molds can be more difficult to fabricate than sphericalones. Aspherical microlenses have been demonstrated as opticaloutcoupling aids for OLED films by Liu et al., Fabrication of anAspherical Microlens for OLED with Modified Etching Process;International Conference on Electrical and Control Engineering 2010.

SUMMARY OF THE INVENTION

In one aspect, a device that includes an organic light emitting devicecomprising an organic emissive layer is provided. An outcoupling layeris optically coupled to the emissive layer, and includes a plurality ofmicrolenses. In an aspect, for each microlens of the plurality ofmicrolenses, each point on a surface of the microlens has a tangentplane that forms an interior angle of not more than 90 degrees with aninterface of the outcoupling layer and the organic light emittingdevice. In an aspect, each microlens of the plurality of microlenses hasa lens height H and a largest base measurement 2R, and H/R is greaterthan 1. In an aspect, H/R is greater than 1.5. In another aspect, H/R isat least 2.

In an aspect, the base measurement R is a radius, and each microlens isradially symmetric about an axis of the microlens that is normal to theinterface of the outcoupling layer with the organic light emittingdevice. In another aspect, each microlens has a rectangular base. In anaspect, each microlens has an elliptical base.

In an aspect, each microlens of the plurality of microlenses has athickness profile defined by a continuous function in r, where r is thedistance from an axis of the microlens centered on the base of themicrolens and normal to the interface of the outcoupling layer with theorganic light emitting device. In an aspect, r is less than or equal toR. In one aspect, the function is a quadratic polynomial of r. Inanother aspect, the function is a cubic polynomial of r.

In an aspect, the outcoupling layer has a periodically varying thicknessacross a plane parallel to the interface of the outcoupling layer withthe organic light emitting device. In one aspect, the plurality ofmicrolenses are closely packed.

In one aspect, the plurality of microlenses comprise a first materialhaving an index of refraction X, and a layer of the organic lightemitting device adjacent to the outcoupling layer comprises a secondmaterial having an index of refraction Y, and the difference between Xand Y is less than 0.1. In an aspect, a layer of the organic lightemitting device adjacent to the outcoupling layer comprises a materialhaving an index of refraction of at least 1.7. In another aspect, theoutcoupling layer comprises a single material. In an aspect, theplurality of microlenses comprise a first material and a portion of theoutcoupling layer adjacent to the organic light emitting devicecomprises a second material. In one aspect, the plurality of microlensescomprise a material having an index of refraction greater than 1.5. Inanother aspect, the plurality of microlenses comprise a material havingan index of refraction greater than 1.7.

In an aspect, the device has a fill factor defined by the fraction of asurface of the light emitting device covered by the plurality ofmicrolenses, and wherein the fill factor is at least about 90%.

In an aspect, at least one microlens of the plurality of microlenses hasa base measurement 2R₁ and at least one microlens of the plurality ofmicrolenses has a base measurement 2R₂, and wherein R₁ is different fromR₂. In another aspect, at least one microlens of the plurality ofmicrolenses has a radius R₁ and at least one microlens of the pluralityof microlenses has a radius R₂, and wherein R₁ is different from R₂. Inone aspect, the plurality of microlenses are arranged in a repeatingpattern.

In an aspect, the outcoupling layer comprises a transparent polymersubstrate. In one aspect, the transparent polymer substrate is flexible.

In an aspect, the plurality of microlenses are embossed into theoutcoupling layer. In one aspect, the plurality of microlenses areformed by a subtractive process. In another aspect, the plurality ofmicrolenses are formed by an additive process.

In an aspect, a method of fabricating an emissive device includesfabricating an outcoupling layer that includes a plurality ofmicrolenses on a first surface of substrate. In one aspect, for eachmicrolens of the plurality of microlenses, each point on a surface ofthe microlens has a tangent plane that forms an interior angle of notmore than 90 degrees with an interface of the outcoupling layer and thesubstrate. In an aspect, each microlens of the plurality of microlenseshas a lens height H and a largest base measurement R, and H/R is greaterthan 1.

In an aspect, the method may further include obtaining an organic lightemitting device, and disposing the organic light emitting device on asecond surface of the substrate opposite the first surface.

In an aspect, the plurality of microlenses are embossed into the lightemissive layer. In one aspect, the plurality of microlenses are formedby a subtractive process. In another aspect, the plurality ofmicrolenses are formed by an additive process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIGS. 3(A) and (B) show an example device and microlens.

FIGS. 4(a), (b), and (c) show an example microlens array.

FIGS. 5(A) and (B) show an example hexagonal close packed array ofmicrolenses and a radial cross section of a microlens.

FIGS. 6(A) and (B) show an example microlens array of incorporating twodifferent sizes of microlenses and a radial cross section of each sizeof microlens.

FIG. 7 shows an example of rays propagating from an array ofmicrolenses.

FIGS. 8(A) and (B) show use of Berrman's method to handle reflectionsfrom the microlens array to air interface onto the OLED structure.

FIGS. 9(A) and (B) show an example of the domain over which the raytracing model simulates outcoupling from microlens structures.

FIGS. 10(A) and (B) show the integrated light extraction of hexagonalclose packed microlens arrays.

FIGS. 11(A), (B), (C), and (D) show the integrated light extraction ofan optimally packed microlens array comprised of two differently sizedmicrolenses.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence is described in moredetail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporatedby reference.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, a cathode 160, and a barrier layer 170.Cathode 160 is a compound cathode having a first conductive layer 162and a second conductive layer 164. Device 100 may be fabricated bydepositing the layers described, in order. The properties and functionsof these various layers, as well as example materials, are described inmore detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which areincorporated by reference.

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is m-MTDATA dopedwith F₄-TCNQ at a molar ratio of 50:1, as disclosed in U.S. PatentApplication Publication No. 2003/0230980, which is incorporated byreference in its entirety. Examples of emissive and host materials aredisclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which isincorporated by reference in its entirety. An example of an n-dopedelectron transport layer is BPhen doped with Li at a molar ratio of 1:1,as disclosed in U.S. Patent Application Publication No. 2003/0230980,which is incorporated by reference in its entirety. U.S. Pat. Nos.5,703,436 and 5,707,745, which are incorporated by reference in theirentireties, disclose examples of cathodes including compound cathodeshaving a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thetheory and use of blocking layers is described in more detail in U.S.Pat. No. 6,097,147 and U.S. Patent Application Publication No.2003/0230980, which are incorporated by reference in their entireties.Examples of injection layers are provided in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety. A description of protective layers may be found in U.S. PatentApplication Publication No. 2004/0174116, which is incorporated byreference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. Pat. No. 7,431,968, which is incorporated by reference in itsentirety. Other suitable deposition methods include spin coating andother solution based processes. Solution based processes are preferablycarried out in nitrogen or an inert atmosphere. For the other layers,preferred methods include thermal evaporation. Preferred patterningmethods include deposition through a mask, cold welding such asdescribed in U.S. Pat. Nos. 6,294,398 and 6,468,819, which areincorporated by reference in their entireties, and patterning associatedwith some of the deposition methods such as ink jet and OVJP. Othermethods may also be used. The materials to be deposited may be modifiedto make them compatible with a particular deposition method. Forexample, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the presentinvention may further optionally comprise a barrier layer. One purposeof the barrier layer is to protect the electrodes and organic layersfrom damaging exposure to harmful species in the environment includingmoisture, vapor and/or gases, etc. The barrier layer may be depositedover, under or next to a substrate, an electrode, or over any otherparts of a device including an edge. The barrier layer may comprise asingle layer, or multiple layers. The barrier layer may be formed byvarious known chemical vapor deposition techniques and may includecompositions having a single phase as well as compositions havingmultiple phases. Any suitable material or combination of materials maybe used for the barrier layer. The barrier layer may incorporate aninorganic or an organic compound or both. The preferred barrier layercomprises a mixture of a polymeric material and a non-polymeric materialas described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporatedby reference in their entireties. To be considered a “mixture”, theaforesaid polymeric and non-polymeric materials comprising the barrierlayer should be deposited under the same reaction conditions and/or atthe same time. The weight ratio of polymeric to non-polymeric materialmay be in the range of 95:5 to 5:95. The polymeric material and thenon-polymeric material may be created from the same precursor material.In one example, the mixture of a polymeric material and a non-polymericmaterial consists essentially of polymeric silicon and inorganicsilicon.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, medical monitors, televisions,billboards, lights for interior or exterior illumination and/orsignaling, heads up displays, fully transparent displays, flexibledisplays, laser printers, telephones, cell phones, personal digitalassistants (PDAs), laptop computers, digital cameras, camcorders,viewfinders, micro-displays, 3-D displays, vehicles, a large area wall,theater or stadium screen, or a sign. Various control mechanisms may beused to control devices fabricated in accordance with the presentinvention, including passive matrix and active matrix. Many of thedevices are intended for use in a temperature range comfortable tohumans, such as 18 degrees C. to 30 degrees C., and more preferably atroom temperature (20-25 degrees C.), but could be used outside thistemperature range, for example, from −40 degree C. to +80 degree C.

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

Microlens arrays are well known outcoupling aids for OLED lighting.Existing examples of microlens arrays feature arrays of microlenses ofuniform size with a spherical thickness profile. According to thepresently disclosed subject matter, improved outcoupling performance canbe achieved by using tall, high aspect ratio microlenses with anaspheric profile. The present disclosure demonstrates that high aspectratio aspheric microlens provide more efficient outcoupling. Accordingto the present disclosure, outcoupling enhancement can also be improvedusing an array of microlenses of different sizes in a repeating patternsuch that smaller microlenses fill gaps between larger ones. Currentmicrolens arrays only feature microlenses of one size. According to thepresently disclosed subject matter, a repeating array of microlenses ofdifferent sizes increases the area of substrate that is covered bymicrolenses, which also improves outcoupling.

In addition, the present disclosure demonstrates that opticaloutcoupling can be improved by fabricating both the microlens array andOLED substrate from high refractive index (e.g., n˜1.7) plastic. Loss oflight due to total internal reflection at the interface between thelight emitting device and substrate can be controlled by using asubstrate that closely matches the index of refraction of the activelayers of the device. OLED materials and indium tin oxide haverelatively high indices of refraction (n˜1.8) compared to conventionalsubstrate materials. Some transparent plastics, such as biaxiallystretched polyethylene napthalate, have a comparable index ofrefraction. OLEDs can be fabricated on substrates with sufficiently highindex to eliminate loss at the device to the substrate interface.Refractive index match between the OLED and the substrate can result inless loss of light within the OLED. The presence of microlenses at thesubstrate-air interface helps to mitigate losses due to the largerefractive index mismatch and promote efficient outcoupling.

Using a high index substrate for OLED growth reduces the loss of lightat the OLED to substrate interface. However, this increases the loss oflight at the substrate to air interface, leading to a net loss of power.According to the present disclosure, forming a microlens array onto theair side of a high index substrate mitigates this loss and results in anet increase in light outcoupling when compared to a lower indexsubstrate with a similar microlens array. Fabricating microlenses froman index matched material added to a planar substrate may yield a devicethat provides similar performance.

Aspherical microlenses with a high aspect ratio H/R can be fabricatedusing the methods such as the one outlined by Kuo et al., Fabrication ofAspherical SU-8 Microlens Array Utilizing Novel Stamping Process andElectrostatic Pulling Method, Optical Society of America 2010. Anordered array of hemispherical droplets of UV curable polymer is stampedonto a substrate. The stamp adds lens material to the substrate; assuch, the lenses and substrate do not necessarily have the samecomposition. The polymer droplets are then stretched in a directionnormal to the substrate using an electric field and take on anapproximately parabolic shape. This shape is then made permanent by UVcuring the droplets. Another technique for making such a structure is toemboss microlens forms directly onto the front surface of a high indexpolymer sheet and then use the back surface of the sheet as an OLEDgrowth substrate.

The presently disclosed outcoupling layer(s), microlens structure(s),and methods of fabricating the same, can be used with OLED lightings anddisplays as well as other types of light emitting devices requiring aplanar substrate. This microlens architecture can also be used forwaveguide outcoupling surfaces, and can also be used for opticalenhancement of backlight outcoupling such as those in LCD displays.

FIG. 3A shows an example device according to an embodiment of thepresent disclosure. According to an embodiment of the presentdisclosure, a device includes an organic light emitting device 300including an organic emissive layer 301, an outcoupling layer 302, andan electrode 309 such as a transparent electrode. The outcoupling layer302 is optically coupled to the emissive layer 301. The outcouplinglayer 302 includes multiple microlenses 303, 304, 305, 306, 307, and308. In an embodiment, the outcoupling layer 302 has a periodicallyvarying thickness across a plane parallel to the interface of theoutcoupling layer 302 with the organic light emitting device 300, suchas the thickness caused by periodic microlenses 303-308.

As shown in FIG. 3B, for a microlens 320, each point on a surface of themicrolens has a tangent plane, such as 321, that forms an interior angle322 of not more than 90 degrees with an interface 323 of the outcouplinglayer and the organic light emitting device. In an embodiment, eachpoint on a surface of the microlens may have a tangent plane, such as321, that forms an interior angle 322 of less than 90 degrees with aninterface 323 of the outcoupling layer and the organic light emittingdevice. As shown in FIG. 3B, a microlens 320 has a lens height H and alargest base measurement 2R. The largest base measurement 2R of amicrolens is the diameter of the microlens. In some embodiments, H/R maybe greater than 1, H/R may be greater than 1.5, and H/R may be at least2. Ray tracing simulations demonstrate that significantly more efficientoutcoupling can be obtained for parabolic and cubic microlenses thanspherical microlenses. Spherical microlenses exhibit optimal outcouplingwhen H/R=1. For H/R<1 outcoupling from parabolic or cubic microlensarrays is inferior to spherical microlenses. However, for H/R>1,improved outcoupling can be obtained for parabolic and cubic arrays thanthat obtained for spherical arrays at H/R=1.

The base of a microlens may be of any shape such as circular,rectangular, square, triangular, elliptical, octagonal, hexagonal, etc.FIG. 3B shows an example microlens according to an embodiment of thepresent disclosure. As shown in FIG. 3B, the base of a microlens may becircular, in which case, the base measurement R is a radius. Forexample, each microlens is radially symmetric about an axis 324 of themicrolens that is normal to the interface 323 of the outcoupling layerwith the organic light emitting device.

An example of a microlens according to the disclosed subject matter is aradially symmetric microlens, as shown in FIG. 3B. Each microlens has athickness profile defined by a continuous function in r, wherein r isthe distance from an axis 324 of the microlens 320 centered on the baseof the microlens 320 and normal to the interface 323 of the outcouplinglayer with the organic light emitting device. As shown, r is less thanor equal to R. The thickness profile of the microlenses can be given byeither equation 10.1, 10.2 or 10.3, and more generally 10.4, shownbelow, where r²=(x−x_(c))²+(y−y_(c))². Equation 10.1 yields microlenseswith a parabolic shape, equation 10.2 is a quadratic polynomial anddescribes microlenses with constant curvature, which are referred to asspherical. Microlenses obeying equation 10.3 are referred to as cubic.

$\begin{matrix}{{{\frac{1}{R^{2}}\left( {x - x_{c}} \right)^{2}} + {\frac{1}{R^{2}}\left( {y - y_{c}} \right)^{2}} + \frac{z}{H} - 1} = 0} & 10.1 \\{{{\frac{1}{R^{2}}\left( {x - x_{c}} \right)^{2}} + {\frac{1}{R^{2}}\left( {y - y_{c}} \right)^{2}} + \frac{z^{2}}{H^{2}} - 1} = 0} & 10.2 \\{{{\frac{1}{R^{2}}\left( {x - x_{c}} \right)^{2}} + {\frac{1}{R^{2}}\left( {y - y_{c}} \right)^{2}} + \frac{z^{3}}{H^{3}} - 1} = 0} & 10.3 \\{{{\frac{1}{R_{1}^{2}}\left( {x - x_{c}} \right)^{2}} + {\frac{1}{R_{2}^{2}}\left( {y - y_{c}} \right)^{2}} + {\frac{1}{H^{n}}z^{n}} - 1} = {{0\mspace{14mu} n} \geq 1}} & 10.4\end{matrix}$

FIGS. 4(a), (b), and (c) show a microlens array with (a) parabolicsurfaces, equation 10.1, (b) spherical surfaces, equation 10.2, and (c)cubic surfaces, equation 10.3. The coordinate system is illustrated inFIGS. 4(a), (b), and (c). Each of the microlenses is centered around itsown origin (x_(c),y_(c)). According to an embodiment, the emissive (top)side of an OLED substrate is patterned with an array of microlenses asshown in FIGS. 4(a), (b), and (c). Coordinates x and y are in plane, andz is out of plane. Each microlens in the array has a symmetry axisrunning through a separate x_(c) and y_(c).

In an embodiment, the multiple microlenses are closely packed. Ingeneral, close-packing of microlenses is a dense arrangement ofmicrolenses in a regular arrangement (or lattice), typically in arepeating pattern. Closely packed refers to the most tightly packed orspace-efficient composition of microlens structures (lattices) that'sallowed by manufacturing techniques. For example, for a hemispheremicrolens system, the neighboring base circles are tangential to eachother. In general, this will be limited by the manufacturing techniquesbeing used. FIG. 5(a) shows a hexagonal close packed array ofmicrolenses and FIG. 5(b) shows a radial cross section of a microlens.The arrangement of microlenses in FIG. 5(a) is an optimally packed arrayof single sized lenses. The microlens radius R and height H are shown inFIG. 5(b) and the thickness profile of this microlens obeys equation10.3 shown above.

Using microlenses with more than 2 different sizes can further improvethe fill factor of microlens, and thus improve light extraction. Inparticular, packing of microlenses can be improved by incorporatingmicrolenses of multiple different sizes into the array as shown in FIGS.6(A) and (B). This improved packing increases the fill factor of themicrolens array, leading to an overall improvement in outcoupling. FIG.6(A) shows an array incorporating two different sizes of microlenses foroptimal area coverage and FIG. 6(B) shows a radial cross section of thetwo different sized microlenses. The large lens radius R_(L) and heightH_(L), and the small lens radius R_(S) and height H_(S) are shown inFIG. 6(B). The thickness profile of the microlenses obey equation 10.3above. As an example, the dimensions of the smaller microlenses are0.6376× that of the larger lenses, although other varying dimensions ofmicrolenses may be used. This improved packing increases the fill factorof the microlens array, leading to an overall improvement inoutcoupling. According to an embodiment, at least one microlens of themultiple microlenses has a base measurement R₁ and at least onemicrolens of the multiple microlenses has a base measurement R₂, and R₁is different from R₂. In the embodiment in which the base measurement Ris a radius, at least one microlens of the multiple microlenses has aradius R₁ and at least one microlens of the multiple microlenses has aradius R₂, and R₁ is different from R₂. In one embodiment, the multiplemicrolenses are arranged in a geometrically repeating pattern, forexample as shown in FIG. 6(A). In an embodiment, the device has a fillfactor defined by the fraction of a surface of the light emitting devicecovered by the multiple microlenses, and the fill factor, i.e., thefraction of the surface of the device covered by microlenses, is greaterthan about 85%, and at least about 90%.

FIG. 7 shows an example of rays propagating from an array of microlenses. When a ray approaches the surface of a microlens, it can refractand pass from the substrate to air or it can reflect back into thesubstrate. These outcomes are governed by Fresnel's equations.Occasionally a ray that is outcoupled from one lens can intersect aneighboring lens and incouple back into the substrate. FIG. 7 showspoints of interaction between rays and the substrate as marked withdots. According to the present disclosure, outcoupling can be furtherenhanced by using a high index substrate to avoid loss of light betweenthe device and substrate. Microlenses help to mitigate the outcouplingloss at the substrate/air interface to allow for a net improvement inoutcoupling. In an embodiment, the multiple microlenses are comprised ofa material having an index of refraction X, and a layer of the organiclight emitting device adjacent to the outcoupling layer is comprised ofa material having an index of refraction Y. In this case, the differencebetween X and Y is less than about 0.1.

In an embodiment, a layer of the organic light emitting device adjacentto the outcoupling layer is comprised of a material having an index ofrefraction of at least about 1.7. For example, referring back to FIG.3A, the layer 301 may be comprised of a material having an index ofrefraction of at least 1.7 In an embodiment, the outcoupling layer iscomprised of a single material. In some cases, the outcoupling layer iscomprised of a transparent polymer substrate. In some cases, thetransparent polymer substrate is flexible. In an embodiment, the multimicrolenses are comprised of a first material and a portion of theoutcoupling layer adjacent to the organic light emitting device iscomprised of a second material. In an aspect, the multiple microlensesmay be comprised of a material having an index of refraction greaterthan about 1.5, greater than about 1.6, or greater than about 1.7.

According to an embodiment, microlens arrays can be fabricated bystamping, in which material is laid down onto a substrate in a pattern.Microlens arrays can be fabricated by embossing, in which material thatis already present on the substrate is formed into a desired surfaceshape using a mold. An array of stamped aspherical microlenses willgenerally require a post-stamping step to form the deposited spots ofmicro lens polymer into the desired shape. Electrostatic stretching ofan array of UV curable polymer microlenses, as demonstrated by Kuo etal. 2010 is an example of such an approach. Stamped polymers generallymust be curable and there are relatively few such high index polymers onthe market. UV curable polymers with refractive indices of n greaterthan 1.6 are available (Morford et al., Press-patterned UV-curable HighRefractive Index Coatings, Society of Photo-Optical InstrumentationEngineers, pp 612301-612301-11 (2006)).

While the embossing process may seem to be conceptually simple, somecare must be taken to generate a metal mold with the appropriatemicrostructure. Once a master with the appropriate microstructure hasbeen generated, a metal mold can be fabricated by electroforming overthe master using the lithography-electroforming-molding (LIGA) process(Becker et al., Fabrication of Microstructures with High Aspect Ratiosand Great Structural Heights by Synchrotron Radiation Lithography,Galvanoforming, and Plastic Moulding (LIGA Process), MicroelectronicEngineering 4 (1986) 35-56). An aspheric lens master can be fabricatedusing a process such as the polymer stamping and electrostaticstretching process described earlier. It should be noted that although amaster must have the correct shape, its optical properties areirrelevant. The master can be stamped onto a conductive substrate to becompatible with the LIGA process. When immersed in a metal plating bath,metal accumulates on the master to form a mold. Regions occupied bypolymer leave voids after the process is complete. The mold is thenready to use after appropriate finishing steps.

According to an embodiment, the multiple microlenses are embossed intothe substrate, such as a polymer substrate. Embossing often refers toanother plastic technique which involves forming a layer of uncuredpolymer pre-coated onto a more robust polymer substrate. Thisfacilitates the formation of a master. Since metal is not required, amold can be cast directly from the master using curable polymer (Liu etal. 2010). Similarly a roller coated with a cured plastic mold can beused to emboss substrates after it has, itself been embossed using amaster. It may be possible to adapt production of aspheric micro lensesfrom curable polymer to roll-to-roll processing using technologydescribed in U.S. Pat. No. 8,257,793 B2 (Forrest 2010).

According to an embodiment of the disclosed subject matter, a method offabricating an emissive device is provided. The method includesfabricating an outcoupling layer that includes multiple microlenses on afirst surface of a substrate. For each microlens among the multiplemicrolenses, each point on a surface of the microlens has a tangentplane that forms an interior angle of not more than 90 degrees with aninterface of the outcoupling layer and the substrate. Additionally, eachmicrolens among the multiple microlenses has a lens height H and alargest base measurement 2R, where H/R is greater than 1. The method mayfurther include obtaining an organic light emitting device, anddisposing the organic light emitting device on a second surface of thesubstrate opposite the first surface. For example, the method offabricating an emissive device may include fabricating an outcouplinglayer that includes multiple microlenses on a first surface of asubstrate prior to depositing an OLED on a second surface of thesubstrate, where the second surface is opposite the first surface. Inone embodiment, the multiple microlenses are embossed into thesubstrate. According to an embodiment, the multiple microlenses may beformed by a subtractive process or by an additive process.

The organic light emitting device described herein may include an anode,a cathode, and an organic emissive layer disposed between the anode andthe cathode. The organic emissive layer may include a host and aphosphorescent dopant.

EXPERIMENTAL

FIGS. 8(A) and (B) shows how Berreman's method is used to modelreflections from the microlens array to air interface onto the OLEDstructure. FIG. 8A shows a stack of thin films constituting the OLEDgrown onto the reverse of a glass substrate. This film includes anindium tin oxide (ITO) anode, the emissive layer of the OLED (EM) andthe aluminum (Al) cathode. FIG. 8(A) also shows incident, reflected, andtransmitted light rays. FIG. 8(B) shows the fraction of light reflectedfrom the layered structure of FIG. 8(A) as a function of incident angle.Light that is not reflected is generally lost due to absorption at thealuminum cathode. Light that is reflected by the OLED structure has asecond opportunity to be outcoupled to air through the microlens array.Therefore a highly reflective OLED structure improves outcouplingefficiency.

A multiple length-scale model was used to estimate the amount of lightvarious microlens architectures could outcouple to air. Berreman'smethod was first used to handle thin film optical effects in the OLEDstructure and determine the angular distribution of light entering thesubstrate. Both forward and backward propagating light from the emissivelayer was considered when solving for the distribution of light into thesubstrate. Light generated by the OLED was assumed to have randompolarization. The results of Berreman's method were used to generate anangular distribution of rays propagating through the substrate using aMonte-Carlo ray tracing algorithm. Rays were introduced at random pointsbeneath a repeating unit of the specific micro lens array architecturesunder study. An example of such a repeating unit, that forms the domainof the ray trace simulation, is shown in FIGS. 9(A) and (B). FIGS. 9(A)and (B) show an example of the domain over which the ray tracing modelsimulates outcoupling from microlens structures. FIG. 9(A) shows a viewnormal to the plane of the substrate. The microlens pattern is brokeninto periodic repeating cells such as the shaded region and a singlecell is chosen as the domain of the simulation. FIG. 9(B) shows thetopography of the microlenses in such a cell. In this case, the cellcontains a pattern of two differently sized micro lenses. Interactionsbetween rays of light and the curved surfaces of the microlens arraywere governed by the Fresnel equations. Transmission and reflectioncoefficients were calculated as an average of σ and π polarizationcases, which depend only on the angle of incident light ray with respectto the normal of plane tangent to the surface where a ray intersects.Light rays in model were allowed to reflect between lens surfaces up to30 times before leaving the simulation. Rays that reflect once off ofthe OLED structure after failing to outcouple to air on their first passthrough the substrate are also considered. An example of rays tracingthrough a simulated microlens array is shown in FIG. 7. Simulationresults are expressed by integrating the strength of light emitted intothe air by the substrate, including both the curved microlenses and flatareas between them, and comparing that value to the light introduced atthe emissive layer.

Berreman's method was also used to compute the direction and strength ofreflection off of the OLED layers for rays of light that do not leavethe substrate on its first pass due to reflection off of themicrolenses. The structure had a reflectivity of between 0.8 and 0.9depending incident angle. The modeled structure and the reflectivity ofthe OLED structure as a function of incident angle are given in FIGS.8(A) and (B). Light that is not reflected is generally lost due toabsorption at the aluminum cathode. Light that is reflected by the OLEDstructure has a second opportunity to be outcoupled to air through themicro lens array. Therefore a highly reflective OLED structure improvesoutcoupling efficiency.

Ray tracing simulations demonstrate that significantly more efficientoutcoupling can be obtained for parabolic and cubic microlenses thanspherical microlenses. Spherical microlenses exhibit optimal outcouplingwhen H/R=1. For H/R<1 outcoupling from parabolic or cubic microlensarrays is inferior to spherical microlenses. However, for H/R>1,improved outcoupling can be obtained for parabolic and cubic arrays thanthat obtained for spherical arrays at H/R=1 as shown in FIGS. 10(A) and(B). FIG. 10 shows the integrated light extraction of hexagonal closepacked micro lens arrays. Light extraction in FIG. 10(A) is presented asa percentage of light generated in the OLED emissive layer that isoutcoupled to air. FIG. 10(A) shows light extraction from a substrateand microlens array with n=1.5. Light extraction in FIG. 10(B) isnormalized to the case of an OLED on a planar substrate of index ofrefraction n=1.5. FIG. 10(B) shows light extraction for the n=1.7 case.Light extraction is plotted as a function of H/R for lens profilesmatching equations 10.1 (square), 10.2 (diamond) and 10.3 (triangle), asdescribed above. The case of spherical lenses with H/R>1 was modeledusing an array of hemispheres on cylindrical pedestals. Ratios of H/R ofup to 10 were tested for various curvatures. Optimal outcoupling forparabolic and cubic arrays is achieved at H/R=2. A 166% enhancement overa plain glass array was achieved for a hexagonal pack of cubic microlenses with index n=1.5 at H/R=2.

Packing of micro lenses can be improved by incorporating lenses of twodifferent sizes into the array as shown in FIGS. 6(A) and (B). Thedimensions of the smaller microlenses are 0.6376× that of the largermicrolenses. This improved packing increases the fill factor of themicrolens array, leading to an overall improvement in outcoupling. FIGS.11(A), (B), (C), and (D) shows the integrated light extraction of anoptimally packed micro lens array comprised of two differently sizedlenses. FIGS. 11(A) and 11(C) show light extraction presented as apercentage of light generated in the OLED emissive layer that isoutcoupled to air, where FIG. 11(A) shows n=1.5 and FIG. 11(C) showsn=1.7. FIGS. 11(B) and (D) show light extraction normalized to the caseof an OLED on a planar substrate. FIG. 11(B) shows light extraction froma substrate and microlens array with n=1.5 and FIG. 11(D) shows lightextraction for with n=1.7. Light extraction was plotted as a function ofH/R for lens profiles matching equations 10.1 (square), 10.2 (diamond)and 10.3 (triangle), as discussed above. As shown in FIGS. 11(A), (B),(C), and (D), a 177% enhancement over a plain glass array is achievedfor cubic two size array microlenses with index n=1.5 at H/R=2. Usingmicrolenses with more than 2 different sizes can further improve thefill factor of microlens, and thus improve light extraction.

Outcoupling can be further enhanced by using a high index substrate toavoid loss of light between the device and substrate. Simulationindicates that an outcoupling of 171% can be achieved by cubic lenseswith H/R=2 for a substrate with index n=1.7 compared with a planarsubstrate of n=1.5. Microlenses help to mitigate the outcoupling loss atthe substrate/air interface to allow for a net improvement inoutcoupling. If the novel, two lens size packing structure is applied,an outcoupling improvement of 187% can be achieved by cubic lenses ofH/R=3 for a substrate with an index of n=1.7.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. The present invention asclaimed may therefore include variations from the particular examplesand preferred embodiments described herein, as will be apparent to oneof skill in the art. It is understood that various theories as to whythe invention works are not intended to be limiting.

The invention claimed is:
 1. A device comprising: an organic lightemitting device comprising an organic emissive layer; and an outcouplinglayer, optically coupled to the emissive layer, comprising a pluralityof microlenses; wherein for each microlens of the plurality ofmicrolenses, each point on a surface of the microlens has a tangentplane that forms an interior angle of not more than 90 degrees with aninterface of the outcoupling layer and the organic light emittingdevice; and wherein each microlens of the plurality of microlenses has alens height H and a largest base measurement 2R, and wherein H/R is atleast 1, and wherein the plurality of microlenses include one or moremicrolenses formed having a dimension of H/R=1 and one or moremicrolenses formed having a dimension of H/R>1.
 2. The device of claim1, wherein H/R is greater than 1.5.
 3. The device of claim 1, whereinH/R is at least
 2. 4. The device of claim 1, wherein the basemeasurement R is a radius, and wherein each microlens is radiallysymmetric about an axis of the microlens that is normal to the interfaceof the outcoupling layer with the organic light emitting device.
 5. Thedevice of claim 1, wherein each microlens has a rectangular base.
 6. Thedevice of claim 1, wherein each microlens has an elliptical base.
 7. Thedevice of claim 1, wherein each microlens of the plurality ofmicrolenses has a thickness profile defined by a continuous function inr, wherein r is the distance from an axis of the microlens centered onthe base of the microlens and normal to the interface of the outcouplinglayer with the organic light emitting device, and wherein r is less thanor equal to R.
 8. The device of claim 7, wherein the function is aquadratic polynomial of r.
 9. The device of claim 7, wherein thefunction is a cubic polynomial of r.
 10. The device of claim 1, whereinthe outcoupling layer has a periodically varying thickness across aplane parallel to the interface of the outcoupling layer with theorganic light emitting device.
 11. The device of claim 1, wherein theplurality of microlenses are closely packed.
 12. The device of claim 1,wherein the plurality of microlenses comprise a first material having anindex of refraction X, and wherein a layer of the organic light emittingdevice adjacent to the outcoupling layer comprises a second materialhaving an index of refraction Y, and wherein the difference between Xand Y is less than 0.1.
 13. The device of claim 1, wherein a layer ofthe organic light emitting device adjacent to the outcoupling layercomprises a material having an index of refraction of at least 1.7. 14.The device of claim 1, wherein the outcoupling layer comprises a singlematerial.
 15. The device of claim 1, wherein the plurality ofmicrolenses comprise a first material and a portion of the outcouplinglayer adjacent to the organic light emitting device comprises a secondmaterial.
 16. The device of claim 1, wherein the plurality ofmicrolenses comprise a material having an index of refraction greaterthan 1.5.
 17. The device of claim 1, wherein the plurality ofmicrolenses comprise a material having an index of refraction greaterthan 1.7.
 18. The device of claim 1, wherein the plurality ofmicrolenses comprise a material having an index of refraction greaterthan 1.6.
 19. The device of claim 1, wherein the device has an fillfactor defined by the fraction of a surface of the light emitting devicecovered by the plurality of microlenses, and wherein the fill factor isat least 90%.
 20. The device of claim 1, wherein at least one microlensof the plurality of microlenses has a base measurement 2R₁ and at leastone microlens of the plurality of microlenses has a base measurement2R₂, and wherein R₁ is different from R₂.
 21. The device of claim 4,wherein at least one microlens of the plurality of microlenses has aradius R₁ and at least one microlens of the plurality of microlenses hasa radius R₂, and wherein R₁ is different from R₂.
 22. The device ofclaim 20, wherein the plurality of microlenses are arranged in arepeating pattern.
 23. The device of claim 1, wherein the outcouplinglayer comprises a transparent polymer substrate.
 24. The device of claim23, wherein the transparent polymer substrate is flexible.
 25. Thedevice of claim 1, wherein the plurality of microlenses are embossedinto the outcoupling layer.
 26. The device of claim 1, wherein theplurality of microlenses are formed by a subtractive process.
 27. Thedevice of claim 1, wherein the plurality of microlenses are formed by anadditive process.
 28. A method of fabricating an emissive devicecomprising: fabricating an outcoupling layer comprising a plurality ofmicrolenses on a first surface of a substrate, wherein for eachmicrolens of the plurality of microlenses, each point on a surface ofthe microlens has a tangent plane that forms an interior angle of notmore than 90 degrees with an interface of the outcoupling layer and thesubstrate; and wherein each microlens of the plurality of microlenseshas a lens height H and a largest base measurement 2R, and wherein H/Ris at least 1, and wherein the plurality of microlenses include one ormore microlenses formed having a dimension of H/R=1 and one or moremicrolenses having a dimension of H/R>1.
 29. The method of claim 28,further comprising obtaining an organic light emitting device, anddisposing the organic light emitting device on a second surface of thesubstrate opposite the first surface.
 30. The method of claim 28,wherein the plurality of microlenses are embossed into the substrate.31. The method of claim 28, wherein the plurality of microlenses areformed by a subtractive process.
 32. The method of claim 28, wherein theplurality of microlenses are formed by an additive process.